I was frustrated that I can't preview instruments using computer keys, but I found the workaround (see answer)
1 Answer
1) For clips, simply press Right Arrow once the clip is selected, so Down Arrow then Right Arrow will preview .alc files
2) For instruments, simply double-click to load, as if you were going to use it. When the instrument loads in, it's highlighted, so you can easily swap it out by double-clicking another browser instrument!
Not as fast as pressing up/down arrows in the browser, but synths are more complicated than wavs
You can even precede the instrument with a C Minor scale (or transpose to A for example), and as long as you have the instrument highlighted, a double click in the browser will replace it, allowing you to experiment in key.